In its classic version, the document contains the names of about 540 Old
Testament persons, each enclosed in a circle or roundel. This Great
Stemma, as I call it, is orders of magnitude larger than the lawyers'
stemma in Isidore's definition or the type of
logical stemma as used by Cassiodorus. The
Great Stemma shows up in medieval compilations of useful historical
documents, as front matter to manuscript bibles of the luxury category
and as a form of illustration to the Commentary on the Apocalypse,
a book written by the Spanish monk Beatus in or about the year 776, but
it is a distinct work of intellectual creativity with its own,
difficult-to-reconstruct history.

This stemma was so prized as a stock book illustration in Spain that it
continued to be used to enrich fine manuscripts for perhaps 500 or 600
years after it was first devised, usually preserving apocryphal persons
and archaic forms of Old Testament names and acquiring an accumulation of
errors.

Some 26 medieval copies of the Great Stemma are documented (two no
longer exist), but it is conceivable that this genealogy of Christ was
reproduced dozens more times. The Great Stemma may have served as model
for the even more popular Genealogy of Christ or Compendium,
a huge scroll-form diagram by Peter of Poitiers or Petrus Pictaviensis.
Echoes of its design are found more than 1,000 years after its first
authorship in engravings supervised by John Speed for the Geneva Bible in
English in 1611[*]Speed.

Although the Great Stemma is termed a "Genealogy of Christ"
(since it reproduces the sum total of Gospel data about Jesus's
ancestry), it bears little resemblance to the graphic genealogies known
as "ascent trees", where the youngest person is the focus and
an ever-widening array of ancestors is shown. Graphically, it is closer
to a "descent tree", which is to say that its ramifications
spring from the ancestors and multiply to their children and
grandchildren. In this sense, it should be more accurately called a
"Genealogy of Adam".

Its root, Adam the first man, is not shown at centre top as a modern
reader might expect, but in the left upper corner, so that the stemma
ramifies both rightwards and downwards. The progress rightwards is at the
macro level, matching a largely lost timeline through thousands of years
from Adam to Christ, while the downwards unfurling is at a micro level,
usually by a single generation and then sibling by sibling.

The graphical conventions used in the Great Stemma are stable enough
that the artist-copyists who sectioned the original large diagram into
pages that would fit in books were able to selectively depart from the
design without descending into nonsense. With the main axes clearly
established, the direction of expansion may exceptionally curl back leftwards
where the artist is short of space to accommodate a very large number of
roundels. The roundels are generally joined by short lines, but where
space is lacking in some of the extant copies, two roundels simply touch
one another to signify that they connect.

Medieval editors must have seen older models that encouraged them to
make the wiring even more complex: the Plutei
stemma adds a long connector that springs from the bottom of one
page to the top of the next, and on the first page of the Millán
stemma, long connectors snake and cross one another rather like a complex
freeway map to exploit all the available page space.

The connector lines themselves are ambivalent. In the upper reaches of
the Great Stemma, they can signify either marriage or a series of
father-son relationships, but where a family is shown in more detail, a
succession of brothers is often entrained along a single vertical or
zig-zagging line. No confusion is caused by this ambiguity, because each
roundel contains the Latin name of both its occupant and his father.

Some scholars argue that the Great Stemma's violations of its own axial
rules imply a lack of sophistication, or that the author lacked the wit
to place similar generations at a similar height on the page, to fork the
connecting lines consistently, or to differentiate the various
relationships (wives, sons or none) with colour coding or special
connectors.[*]See in particular the
criticism by Klapisch-Zuber, Christiane, L'Ombre des Ancêtres.

Those defects are more likely in fact to be damage done by editors to
the archetypal stemma. Even if some of these "illogical" features do stem
from the original format, they can hardly be taken as evidence that the
Great Stemma's anonymous author was incompetent at graphic design. Some
of the features follow design rules that have their own logic, though
this may not be evident to modern readers, and others reflect what we
would nowadays call "loose" document standards in the sense
that most hand-designed web pages in the late 20th century employed
"loose" hypertext markup language (HTML), not
"strict" markup, or that manuscript documents up to the 19th
century, even by writers with sophisticated orthographic training, were
liberal in their spelling.

Loose solutions are not proof of incompetence, but indicators that the
authors or editors perceived over-strictness as a drain on time and a
distraction from their mission of writing for clarity and immediacy.

The original practical purpose of the Great Stemma is not entirely
clear. It is definitely not a mnemonic device, a two-dimensional theatre
of memory, as has sometimes been suggested. Complex tree-form
diagrams reduce content to a logical minimum and are devoid of any
accompanying markers that might nurture visual memory. Genealogies are
far easier to learn in the form of narratives or lists. Even copying the
Great Stemma to a blank page in a scriptorium was difficult to do
accurately, as the numerous errors in our copies prove. Like a detailed
electrical wiring diagram, a stemma is unsuitable as a device for
inputting information to the human memory.

In cognitive terms, a stemma is most useful as as a mechanism for mental
output and for indexing what one knows in a ready reference. Such a
diagram can be termed an "external storage device". As
Cassiodorus might have said, a stemma may either be used to format the
mind to learn from the verbal explanation that follows, or allow one to
test later what has been memorized, but it cannot replace other learning
methods including classroom discourse.

If the author of the Great Stemma did intend to create such a monument
of visual analysis, his editors soon subverted his purpose. They built
more and more errors into the Great Stemma, yet failed in their efforts
at revision, leaving the biblical names in outdated Latin or corrupted
forms that were unfindable in the Vulgate Latin. As a work of reference
it became not only confusing but unusable, yet still it kept on being
reproduced. Why?

In its day, the Great Stemma would have appeared to many as a masterly
scientific reformulation of scripture. Unlike difficult text or easy
pictorial illuminations, it would have impressed many readers as an
intellectually rigorous distillation into 16, 14 or fewer pages of the
story of the Incarnation and Salvation. A user was not required to read
it from start to finish, but could simply appreciate it as a whole,
fixing an eye on the well-known root persons - Adam, Noah, Abraham and
David - while sampling the exotic names from just a few randomly chosen
roundels.

The images at the page corners of some editions and the panels of text
between the roundels served somewhat like pictures and captions today,
inviting the reader to focus on them, then simply fly over the rest of
the content.

We cannot be certain when the Great Stemma was compiled, or by whom, or
even if its original language was Latin or Greek, but it seems to be
beyond doubt that a single, now-lost model circulating in Visigothic
Spain accounts for its wide distribution there. There is no evidence that
the Great Stemma was new in the 9th century: there is a great deal of
circumstantial evidence that it was by then already very old.

The compelling evidence that the Great Stemma dates back to Late
Antiquity comes from an analysis of
its structure and comparison with another document, the Liber
Genealogus, which can be reliably dated to the year 427. The
Great Stemma's theological content
shows an affinity with the work of early Christian writers. Extensive
errors in the text in the medieval period had arisen through copying and
recopying, without access to the original text.

The Great Stemma's left-to-right orientation suggests that it was
originally drawn in one piece, on a scroll to be unrolled from left to
right, and was only later split into 18 or fewer discrete codex pages.
The way that certain errors
collectively occur across page breaks does indeed suggest that certain
mistakes might have been made while the Great Stemma was still all in one
piece.